ODP488 Incidental papillary thyroid cancer found after hemi-thyroidectomy in man with MUTYH-associated polyposis

Abstract MUTYH-associated polyposis (MAP) is an autosomal recessive syndrome caused by a mutation to the MUTYH gene, a base excision repair gene that repairs oxidative damage to DNA. Mutations to both allelic copies of this gene typically manifest as multiple colonic polyps, which develop by middle age. Individuals with MAP are also at higher risk for developing non-gastrointestinal (GI) cancers including those affecting the ovary, bladder, breast, endometrium, and possibly thyroid. A 66-year-old man with a history of MAP complicated by colonic polyposis requiring subtotal colectomy, an esophageal adenocarcinoma resected by esophagectomy, and a gastrointestinal stromal tumor also resected was referred to endocrinology clinic for an incidental thyroid nodule found on routine surveillance. The patient denied prior neck radiation but reported having a daughter and cousin with thyroid cancer. Ultrasound showed a left 3.3 by 2.8 by 1.6 cm thyroid nodule with multiple lobulations. Although a fine needle aspiration (FNA) of the nodule was benign, a repeat ultrasound several months later noted an increase in size to 3.5 by 3.5 by 2. 0 cm. Given the nodule's significant growth, the patient's history of MAP, and family history of thyroid cancer, the patient underwent a left hemi-thyroidectomy. Pathology revealed that the nodule was a follicular adenoma, however also identified two papillary microcarcinomas, measuring 1 and 2 mm. The patient was started on levothyroxine 50 mcg with a goal TSH below 0.5 mU/L and was monitored with ultrasounds which were unremarkable. In 2021, the national cancer institute estimated approximately 150,000 cases of colorectal cancer (CRC) with a 65% 5-year survival, compared to 44,000 cases of thyroid cancer with a 98% 5-year survival. Although the incidence of CRC in patients with MAP is high, approximately 43-63% by age 60, the risks of several associated non-GI malignancies are not as well defined. The national comprehensive cancer network (NCCN) and the American College of Gastroenterology (ACG) both published guidelines regarding appropriate screening for both GI and associated non-GI malignancies. In regards to thyroid cancer, both advise screening in patients with familiar adenomatous polyposis (FAP), another familial condition, due to the increased risk of papillary thyroid cancer (PTC) of approximately 1-2%. The evidence concerning MUTYH is not as strong, with the ACG recommending annual thyroid cancer screening, although the quality of evidence is low. Since a thyroid ultrasound is both low-cost and non-invasive and the overall survival of patients with PTC is high, we agree that patients with MAP be screened with thyroid ultrasounds. This case highlights the possibility that thyroid cancer is a manifestation of MAP, and a lower threshold to send for surgery may be warranted for those with other risk factors such as a family history or prior neck radiation. Presentation: No date and time listed